When two countries share a border, it sometimes occurs that occupants of one side wish to surreptitiously cross the border, either to carry on forbidden trade, or to immigrate, or to conduct clandestine military operations. These crossings are sometimes done by digging tunnels under the border from one side to the other. In such cases the entered country may wish to prevent such surreptitious entry. Interlopers may also dig tunnels to gain entry to sensitive installations such as electric grid facilities. Such entry can be prevented by installing an underground electric fence. The fence can also be used to protect against entry into sensitive installations such as electric power stations.
Constructing such a fence may be a more peaceful way of preventing such illicit activity than invading the foreign territory to destroy existing tunnels.
Similarly, a non-contact electric fence may be installed above ground. Anything, including land, a building or a ship, could be protected by the electric fence.
Electric fences are well known in air, in the form of cattle fences. A pulse of voltage is applied periodically, at a potential sufficient to deter contact with the fence by people or animals, but less than lethal. Fishermen know that by placing a metal electrode in the ground and energizing with the positive terminal from a household electric outlet, earthworms can be made to wriggle to the surface and be collected for bait. These applications use low frequency current, which is carried by water containing a small amount of dissolved salts. The worms are in contact with the soil and become part of the circuit. At least one person has been killed by walking on the ground near a power transmission line that accidently was grounded. People have been killed by swimming near boats that are leaking current from connected shore power.
Pulses may be generated by storing electric energy in capacitors or inductors, and releasing the energy across the electrodes suddenly. The circuit is similar to that used for an automobile spark plug. A direct current is caused to flow at a relatively low voltage in the primary winding of a coil, and when the primary switch is opened, a spark jumps a gap in the secondary at high potential. Alternatively, a capacitor can be charged and suddenly released, as is done in a method that replaces dynamite in treating oil wells (Blue Spark Inc. website.) The pulse also emits electro-magnetic radio waves, unless these are damped by a capacitor. In fact such pulses were the first method to transmit radio communications.
Electric fences have been used in water to prevent migration of undesired fish up a river (DV Smith, 1995.) Their method also makes use of pulses.
We have not found previous attempts to provide an electric fence in soil. However, Vinsome et al 1994 passed low frequency (60 Hz) current between pipes in a pattern of wells in order to produce oil. The electric current stops flowing when the soil dries out. In desert regions there may be little moisture to begin with.
Radio-frequency (RF) can penetrate soil better than low frequency. Rowland and Newton 1980 patented an RF antenna placed in a borehole to heat the surrounding earth and produce oil from oil shale. Like a microwave oven, the heating does not require a conducing path, but depends on exciting vibrations of polar molecules, even traces of water. The design was based on radio communication antennas that operate in air, and did not work well in the earth, which absorbs RF more strongly than air. Their antenna produced an uneven pattern of heating.
Bridges et al 1979 revealed an array of three rows of RF electrodes to more uniformly heat the deposit between the rows. The array was tested on a pilot scale first on oil shale and later on oil sands (Carlson et al 1981.) Later Dev et al 1984 and 1988 described use of the method to remediate hazardous waste sites, which they successfully heated by such arrays of vertical electrodes placed in boreholes.
In treating one of the hazardous waste sites Bridges et al 1987 and Dev et al 1988 heated the soil by placing RF electrodes on the surface of the ground, in a pattern of alternating positive and negative electrodes. The soil was heated for a depth of a few meters by fringing fields from the electrodes. Heating to greater depths was not achieved, and a row of vertical electrodes was not tried.
The potential hazards associated with RF electromagnetic fields are discussed in the FCC's OET Bulletin No. 56.